TR.28.2.1 Soft Story Checking
STAAD.Pro will perform this check when the option CHECK SOFT STORY command is used following a floor diaphragm. If omitted, no soft story check is performed. Soft story checking is only valid for structures having vertical elements in the form of columns and shear wall (without opening). When used, the program will exclude the effect of any other forms of lateral force resisting structural elements.
Description
The option check for soft story can be performed per the IS 1893:2002, IS 1893-2016, or ASCE 7-95 codes.
A soft story building is a multi-story building where one or more floors are soft due to structural design. These floors can be dangerous in earthquakes, because they cannot cope with the lateral forces caused by the swaying of the building during a quake. As a result, the soft story may fail, causing what is known as a soft story collapse.
Soft story buildings are characterized by having a story which has a lot of open space. Parking garages, for example, are often soft stories, as are large retail spaces or floors with a lot of windows. While the unobstructed space of the soft story might be aesthetically or commercially desirable, it also means that there are less opportunities to install shear walls, specialized walls which are designed to distribute lateral forces so that a building can cope with the swaying characteristic of an earthquake.
If a building has a floor which is 70% less stiff than the floor above it, it is considered a soft story building. This soft story creates a major weak point in an earthquake, and since soft stories are mostly associated with retail spaces and parking garages, they are often on the lower stories of a building, which means that when they collapse, they can take the whole building down with them, causing serious structural damage which may render the structure totally unusable.
When used, the program checks for soft stories per Clause 7.1 Table 5 i)a and i)b for IS 1893 and per Clause 12.3.3 Table 12.3-2 1a and 1b for ASCE 7-05 (used for seismic design categories D, E, and F).
Stiffness Irregularities
- Stiffness Irregularities: Soft Story – As per this provision of the code, a soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average lateral stiffness of the three story above.
- Stiffness Irregularities: Extreme Soft Story – As per
this provision of the code, a extreme soft story is one in which the lateral
stiffness is less than 60 percent of that in the story above or less than 70
percent of the average lateral stiffness of the three story above.
Thus, if any story of a building is found to be soft or extremely soft, the building is likely to suffer much damage in an earthquake than a similar type of building but has more regular vertical stiffness.